A screened background is currently used for complicating the falsification of documents by scratching and substituting indications. But known screening techniques have drawbacks: either the screen is simple and very visible and in this case may be reconstituted or else it has a level of complexity which makes it practically impossible to check its condition by a simple visual examination of the document.
There exist a number of printing methods employing dot pattern matrices with only some density levels available for achieving printing with many grey levels, which however do not help in reducing the risk of tampering: reference may for instance be made to "New grey scale printing method using a thermal printer" in IEEE Transactions on Electron Devices, Vol. ED30 (1983), Aug. No. 8, New York.